The present invention relates to a hydrocarbon conversion process. More particularly, this invention relates to the catalytic hydrocracking of hydrocarbons.
The hydrocracking of hydrocarbons is old and wellknown in the prior art. Hydrocracking processes can be used to hydrocrack various hydrocarbon fractions such as reduced crudes, gas oils, heavy gas oils, topped crudes, shale oil, coal extract and tar extract wherein these fractions may or may not contain nitrogen compounds. Modern hydrocracking processes were developed primarily to process feeds having a high content of polycyclic aromatic compounds, which are relatively unreactive in catalytic cracking. The hydrocracking process is used to produce desirable products such as turbine fuel, diesel fuel, and middle distillate products such as naphtha and gasoline.
The hydrocracking process is generally carried out in any suitable reaction vessel under elevated temperatures and pressures in the presence of hydrogen and a hydrocracking catalyst so as to yield a product containing the desired distribution of hydrocarbon products.
Hydrocracking catalysts generally comprise a hydrogenation component on an acidic cracking support. More specifically, hydrocracking catalysts comprise a hydrogenation component selected from the group consisting of Group VIB metals and Group VIII metals of the Periodic Table of Elements, their oxides or sulfides. The prior art has also taught that these hydrocracking catalysts contain an acidic support comprising a crystalline aluminosilicate material such as X-type and Y-type aluminosilicate materials. This crystalline aluminosilicate material is generally suspended in a refractory inorganic oxide such as silica, alumina, or silica-alumina.
Regarding the hydrogenation component the preferred Group VIB metals are tungsten and molybdenum the preferred Group VIII metals are nickel and cobalt. The prior art has also taught that combinations of metals for the hydrogenation component, expressed as oxides and in the order of preference, are: NiO-WO.sub.3, NiO-MoO.sub.3, CoO-MoO.sub.3, and CoO-WO.sub.3. Other hydrogenation components broadly taught by the prior art include iron, ruthenium, rhodium, palladium, osmium, indium, platinum, chromium, vanadium, niobium, and tantalum.
As can be appreciated from the above, there is a myriad of catalysts or catalyst systems known for hydrocracking whose properties vary widely. A catalyst suitable for maximizing naphtha yield may not be suitable for maximizing the yield of turbine fuel or distillate. Further, the various reactions; i.e., denitrogenation, hydrogenation, and hydrocracking must be reconciled in a hydrocracking process in an optimum manner to achieve the desired results.
For instance when a feedstock having a high nitrogen content is exposed to a hydrocracking catalyst containing a high amount of cracking component the nitrogen serves to poison or deactivate the cracking component. Thus, hydrodenitrogenation catalysts do not possess a high cracking activity since they are generally devoid of a cracking component that is capable of being poisoned. Another difficulty is presented when the hydrocracking process is used to maximize naphtha yields from a feedstock containing light catalytic cycle oil which has a very high aromatics content. The saturation properties of the catalyst must be carefully gauged to saturate only one aromatic ring of a polynuclear aromatic compound such as naphthalene in order to preserve desirable high octane value aromatic-containing hydrocarbons for the naphtha fraction. If the saturation activity is too high, all of the aromatic rings will be saturated and subsequently cracked to lower octane value paraffins.
On the other hand, distillate fuels such as diesel fuel or aviation fuel have specifications that stipulate a low aromatics content. This is due to the undesirable smoke production caused by the combustion of aromatics in diesel engines and jet engines.
Prior art processes designed to convert high nitrogen content feedstocks are usually two stage processes wherein the first stage is designed to convert organic nitrogen compounds to ammonia prior to contacting with a hydrocracking catalyst which contained a high amount of cracking component; e.g., a molecular sieve material.
For instance U.S. Pat. No. 3,923,638 to Bertolacini et al. discloses a two catalyst process suitable for converting a hydrocarbon containing substantial amounts of nitrogen to saturated products adequate for use as jet fuel. Specifically, the subject patent discloses a process wherein the hydrodenitrogenation catalyst comprises as a hydrogenation component a Group VIB metal and group VIII metal and/or their compounds and a cocatalytic acidic support comprising a large-pore crystalline aluminosilicate material and refractory inorganic oxide. The hydrocracking catalyst comprises as a hydrogenation component a Group VIB metal and a Group VIII metal and/or their compounds, and an acidic support of large-pore crystalline aluminosilicate material. For both hydrodenitrogenation catalyst and the hydrocracking catalyst, the preferred hydrogenation component comprises nickel and tungsten and/or their compounds and the preferred large-pore crystalline aluminosilicate material is ultrastable, largepore crystalline aluminosilicate material.
The prior art has also generally disclosed that the activity of a catalyst can be increased by decreasing the particle size of a catalyst. For instance, U.S. Pat. No. 3,857,780 (Gustafson) teaches that a reduction in catalyst particle size increases the activity of hydroforming catalysts. U.S. Pat. No. 3,796,655 (Armistead et al.) discloses a hydrodesulfurization process in which a small particle size catalyst is utilized to increase catalyst activity. Further, this patent recognizes that decreasing catalyst size (1/16-inch to 1/32-inch) while maintaining all other parameters constant, e.g., reactor dimensions and space velocity, results in a pressure drop increase that reduces the catalyst activity advantage. Similarly, U.S. Pat. No. 3,563,886 (Carson et al.) discloses a hydrddesulfurization process wherein the catalyst has increased activity and a particle size diameter between 1/20-inch and 1/40-inch. This patent presents data relating reactor pressure drop to catalyst particle size and reactor diameter. Thus, the prior art has recognized that this increase in activity afforded by a reduction in catalyst size is offset by an increase in the pressure gradient through the reaction system. Hence any catalyst system implementing this increase in activity phenomenon must also display a tolerable pressure gradient.
The various prior art hydrocracking processes do not exploit distinctions in catalyst particle size in any discernible manner. U.S. Pat. No. 4,120,825 (Ward) discloses a denitrogenation-hydrocracking process that utilizes a catalyst containing zeolitic aluminosilicates, alumina, and at least one of the metals, oxides and sulfides of Groups VIB and VIII. This catalyst as disclosed in Example 1 was formed into 1/16-inch extrudates.
U.S. Pat. No. 4,689,137 (Clark) discloses a hydrocracking catalyst containing a crystalline aluminosilicate zeolite ion-exchanged with rare earth cations and Group VIII noble metal cations in combination with a porous, inorganic refractory oxide. Example I shows that this catalyst was in a particulate form consisting of 1/8-inch extrudate.
U.S. Pat. No. 3,431,196 (Dobres) discloses a hydrocracking catalyst that contains nickel and/or cobalt in combination with a Z-14 U.S. zeolite and silica-alumina. Example 1 shows that the catalyst support granules were sized to obtain particles that are retained on a 24 mesh sieve and that passed through a 14 mesh sieve (U.S. Sieve).
U.S. Pat. No. 3,923,638 (Bertolacini et al.), mentioned above, in Example I shows a hydrodenitrogenation catalyst that passes through a 20-mesh sieve (U.S. Sieve), but not a 40-mesh sieve (U.S. Sieve). Example II discloses a hydrocracking catalyst that passes through a 12-mesh sieve (U.S. Sieve) but not a 20-mesh sieve (U.S. Sieve).
U.S. Pat. Nos. 4,576,711, 4,563,434, and 4,517,073 all to Ward et al. disclose in Example I a hydrocracking catalyst that has been extruded in a die having openings between 1/32 and 1/8 inch wherein the extruded material is subsequently cut into lengths of about 1/32 to 3/4 inch preferably 1/4 to 1/2 inch. Example II discloses hydrocracking particulate catalysts having a three-leaf clover cross-sectional shape with a diameter between 0.02 and 0.04 inches and a length between 1/4 and 1/2 inches.
U.S. Pat. No. 3,649,523 (Bertolacini et al.) discloses hydrocracking catalysts that pass through a 12-mesh sieve (U.S. Sieve) but not a 20-mesh sieve (U.S. Sieve).
The prior art does not disclose or suggest that the selectivity of a hydrocracking process towards naphtha can be improved by using disparate catalyst particle sizes in a plurality of reaction zones in series, wherein a reaction zone containing catalyst having a relatively small nominal particle size is situated downstream of a reaction zone containing catalyst having a greater particle size. The hydrocracking process of the present invention provides a substantial naphtha selectivity advantage and concomitantly affords greater catalyst activity with less light gas production while not deleteriously increasing the pressure gradient across the reaction zones.